1. Field of the Invention
This invention relates generally to the field of semiconductor fabrication. More specifically, the invention relates to a method of forming a ruthenium containing film on a substrate.
2. Background of the Invention
Ruthenium and ruthenium compounds such as ruthenium oxide are materials considered to be promising for use as capacitor electrode materials in the next generation DRAMs. High dielectric constant materials (aka high-k materials) such as alumina, tantalum pentoxide, hafnium oxide, and barium-strontium titanate (BST) are currently used for these capacitor electrodes. These high-k materials, however, are produced using temperatures as high as 600° C., which results in oxidation of polysilicon, silicon, and aluminum and causes a loss of capacitance. Both ruthenium and ruthenium oxide, on the other hand, exhibit a high oxidation resistance and high conductivity and are suitable for application as capacitor electrode materials. They also function effectively as oxygen diffusion barriers. Ruthenium has also been proposed for the gate metal for lanthanide oxides. In addition, ruthenium is more easily etched by ozone and by a plasma using oxygen than are platinum and other noble metal compounds. The use of ruthenium as a barrier layer separating low-k material from plated copper and as a seed layer has also been attracting attention recently.
High-quality films of ruthenium and ruthenium oxide (RuO2) can be deposited under appropriate conditions from a precursor of high-purity ruthenium tetroxide (RuO4). This precursor can also be used for the deposition (film formation) of perovskite-type materials, such as strontium ruthenium oxide, that exhibit an excellent conductivity and a three-dimensional structure very similar to that of barium-strontium titanate and strontium titanium oxide.
However, high-purity ruthenium tetroxide, is a strong oxidizing agent, and is considered to have a high toxicity. In addition, high-purity ruthenium tetroxide has a boiling point of about 130° C. and therefore it presents an explosion risk at high temperatures (above about 108° C.). It is therefore recommended that pure ruthenium tetroxide be stored at low temperatures in order to avoid its potential for decomposition (explosion).
Given these properties of ruthenium tetroxide (RuO4) (particularly the explosion risk during holding), when used as a reactant it is normally necessary to use it diluted in an appropriate solvent. Water, carbon tetrachloride, and alkanes, for example, are known for use as this solvent.
In the case of water as a solvent it is further necessary to add a stabilizer such as NalO4 in order to prevent RuO4 from reacting and decomposing during holding. Further, the use of such an aqueous RuO4 solution as a ruthenium based precursor may result in the introduction of impurities into the film and the tool (e.g., reaction chamber).
The electronics industry is abandoning carbon tetrachloride due to its high toxicity; therefore it becomes a less than ideal solvent choice for a ruthenium tetroxide precursor solution.
Alkanes such as pentane and octane are can be used as solvents for RuO4, but the reaction between the alkane (for example, pentane) and RuO4 causes the incorporation of carbon when alkane containing dissolved RuO4 is used as a ruthenium based precursor in film production. Carbon causes an increase in the resistance of ruthenium-type films, and as a consequence the presence of carbon during film production can be seen as less than ideal.